The system of acupuncture can be traced back to China and has probably been practiced for thousands of years. The therapy became an international crossover hit much more recently. In 1972 President Richard M. Nixon visited the People's Republic, and the Western journalists accompanying him described spectacular successes of acupuncture in anesthesia and pain control--including cesarean sections in which the women got only needles and no anesthetics.

Even as acupuncture has grown in popularity, though, it has never been able to shake suspicions that it is merely an esoteric sham treatment. Over the years, researchers have discovered that the simple act of placing a needle through the skin does seem to benefit some patients and trigger a surprising array of physiological activity. But it is also clear that the technique may have powerful placebo effects that could explain its success. Sorting out the relative contributions of neurobiology and psychology to acupuncture's power has become the yin and yang for researchers in this field.